Optical glass fibers are usually coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which contacts the glass surface is called the inner primary coating and the overlaying coating is called the outer primary coating.
The inner primary coating is usually a soft coating having a low glass transition temperature (hereinafter "Tg"), to provide resistance to microbending. Microbending can lead to attenuation of the signal transmission capability of the coated optical glass fiber and is therefore undesirable. The outer primary coating is typically a harder coating providing desired resistance to handling forces, such as those encountered when the coated fiber is cabled.
For the purpose of multi-channel transmission, optical glass fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical glass fiber assemblies include ribbon assemblies and cables. A typical optical glass fiber assembly is made of a plurality of coated optical glass fibers which are bonded together in a matrix material. For example, the matrix material can encase the optical glass fibers, or the matrix material can edge-bond the optical glass fibers together.
Optical glass fiber assemblies provide a modular design which simplifies the construction, installation and maintenance of optical glass fibers by eliminating the need to handle individual optical glass fibers.
Coated optical glass fibers for use in optical glass fiber assemblies are usually coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to facilitate identification of the individual coated optical glass fibers. Such ink coatings and colored outer primary coatings are well known in the art. Thus, the matrix material which binds the coated optical glass fibers together contacts the outer ink layer if present, or the colored outer primary coating.
When a single optical glass fiber of the assembly is to be fusion connected with another optical glass fiber, or with a connector, an end part of the matrix layer can be removed to separate each of the optical glass fibers.
Desirably, the primary coatings on the coated optical glass fibers, and the ink coating if present, are removed simultaneously with the matrix material to provide bare portions on the surface of the optical glass fibers (hereinafter referred to as "ribbon stripping"). In ribbon stripping, the matrix material, primary coatings, and ink coating, are desirably removed as a cohesive unit to provide a clean, bare optical glass fiber which is substantially free of residue. This residue can interfere with the optical glass fiber ribbon mass fusion splicing operation, and therefore usually must be removed by wiping prior to splicing. However, the step of removing the residue can cause abrasion sites on the bare optical glass fiber, thus compromising the strength of the connection. The superior stripping functionality of ribbon assemblies to provide clean, residue-free, bare optical glass fibers during ribbon stripping according to this invention has heretofore been believed to be unobtainable.
A common method for practicing ribbon stripping at a terminus of the ribbon assembly is to use a heated stripping tool. Such a tool consists of two plates provided with heating means for heating the plates to about 90 to about 120 C. An end section of the ribbon assembly is pinched between the two heated plates and the heat of the tool softens the matrix material and the primary coatings on the individual optical glass fiber. The heat-softened matrix material and heat-softened primary coatings present on the individual optical glass fibers can then be removed to provide bare optical glass fiber ends, at which the fusion connections can be made. A knife cut is often used to initiate a break in the matrix material to the inner primary coating. Typically, only about a 1 to 4 cm section of the matrix material and coatings on the optical glass fibers need be removed. Identification of the bare individual optical glass fibers achieved by tracing back along the bare optical fiber until the ink coating or colored outer primary coating is seen.
U.S. Pat. No. 5,373,578 discloses a ribbon assembly containing a plurality of coated optical glass fibers. Each of the optical glass fibers is coated with an inner primary coating which is adjacent to the optical glass fiber, with an outer primary coating and an ink coating on the outer primary coating. The inner primary coating is modified so that adhesion between the inner primary coating and the optical glass fiber is reduced. This reduction in adhesion facilitates easy removal of the heat-softened primary coating when using a heat stripping method. While this patent discloses, at column 5, lines 10-13, that the adhesion between the inner primary coating and the optical glass fiber should be sufficient to prevent delamination of the inner primary coating from the optical glass fiber, any reduction in the adhesion between the inner primary coating and the optical glass fiber increases the possibility of such undesirable delamination, especially in the presence of moisture. Delamination of the inner primary coating from the optical glass fiber can lead to degraded strength of the optical glass fiber as well as signal transmission attenuation disadvantages.
Published European patent application 0262340 discloses a ribbon cable having a "peel layer" as the outermost coating layer on each of optical glass fibers contained within the ribbon cable. During ribbon stripping, the peel layer is destroyed and the matrix material is removed from the coated optical glass fibers. However, after ribbon stripping, the optical glass fibers are still coated with the primary coatings. That is, the primary coatings are not simultaneously removed with the matrix material in the ribbon assemblies disclosed in this publication.
U.S. Pat. No. 5,011,260 discloses a ribbon cable having a "decoupling layer" disposed between the coated optical glass fibers and the matrix material. In this manner, the matrix material may be easily removed from the coated optical glass fibers by application of low stripping force. This patent includes a general statement that the coatings on the optical glass fiber can be simultaneously removed with the matrix material during ribbon stripping. However, this patent fails to teach how to solve the problems associated with the residues remaining on the bare optical glass fibers after ribbon stripping conventional ribbon assemblies.
Published European patent application 0407004 discloses a ribbon cable containing a matrix material having sufficient adhesion to the ink coated optical glass fibers to remain adhered thereto during normal use but is easily strippable therefrom without damaging the integrity of the ink layer on the coated optical glass fibers. Thus, the ribbon assembly disclosed in this publication does not have the capability of removing the primary coatings on the optical glass fibers simultaneously with removal of the matrix material during ribbon stripping, so as to provide residue-free bare optical glass fibers.
Published European patent application 0527266 discloses a ribbon cable containing a lubricating "interfacial layer" which separates the matrix material from the coated optical glass fibers. The interfacial layer facilitates easy removal of the matrix material from the coated optical glass fibers. While this publication discloses at page 3, line 15, that the buffer layer and first protective coating can be stripped in one step, there is no disclosure teaching how to accomplish such an operation. Furthermore, the lubricating interfacial layer will inhibit simultaneous removal of the first protective coating with the matrix material. Thus, this publication does not teach how to make a ribbon assembly having the capability of removing the primary coatings on the optical glass fibers simultaneously with the matrix material during ribbon stripping, so as to provide residue-free bare optical glass fibers.
U.S. Pat. No. 4,900,126 discloses a ribbon cable in which the bonding adhesive forces between the ink layer and the primary coatings on the optical glass fibers are greater than the bonding between the ink layer and the matrix material. In this manner, the matrix material can be easily removed from the ink coated optical glass fibers without removing the ink layer. However, this patent does not address the problems associated with removing the primary coating layers simultaneously with the matrix material.
U.S. Pat. No. 4,660,927 teaches a silicone-coated optical fiber in which the soft silicone coating is easily peeled from the surface of the optical glass fibers by finger pressure. The coating contains a first siloxane component having aliphatic unsaturated groups and a second siloxane component having mercaptoalkyl groups. Because such a coating is easily peelable, as by rubbing with finger pressure, the coating has insufficient adhesion to the surface of the optical glass fibers to prevent delamination during most uses. Furthermore, this patent does not address the problems of ribbon stripping, but rather only the stripping of a single optical glass fiber. It is generally known that three coating systems (inner primary coating, outer primary coating, and ink coating) having acceptable single fiber strippability will exhibit dramatically different levels of strippability characteristics when used in ribbon form.
U.S. Pat. No. 4,496,210 provides a radiation-curable optical fiber coating composition containing a polysiloxane. However, this patent does not address the problems associated with ribbon stripping.
Japanese Patent Application H3-35210 teaches to combine a liquid lubricant, such as liquid silicone oil or liquid aliphatic oil, with a mercaptosilane compound in an inner primary coating composition. During stripping, when the bond between the surface of the optical glass fiber and inner primary coating is broken the liquid lubricant invades the boundary between the surface of the optical glass fiber and the inner primary coating. The liquid lubricant must not have a high compatibility with the inner primary coating or it will not bleed out of the inner primary coating during stripping. However, this document fails to teach a system to adjust the level of fiber friction between the adjacent surfaces of the optical glass fiber and the inner primary coating to a level which provides a resistive force that is less than the cohesive strength of the inner primary coating. Thus, while this document teaches that the inner primary coating can be stripped more easily by incorporating liquid lubricant compounds, the inner primary coating will still leave unwanted residue on the surface of the optical glass fiber if the above described fiber friction forces are at a level which provide a resistive force that is greater than the cohesive strength of the inner primary coating.
One primary coating composition available from JSR Corporation, designated as R-1055, is specified as having, inter alia, a viscosity of 5000 cps @25.degree. C., a glass transition temperature of -4.degree. C., a shrinkage value of 2.9%, a tensile strength value of 0.21 kg/mm.sup.2, a tensile elongation value of 195%, an adhesion force of 20 g/cm and a Young's modulus @23.degree. C. of 0.12 kg/mm.sup.2. When this composition was tested in accordance with the test methods herein, it had a measured crack propagation value of 1.56 mm (standard deviation 0.2), and a fiber pull-out friction value of 26.3 g/mm (standard deviation 1.65).
There are many test methods which may be used to determine the performance of a ribbon assembly during ribbon stripping. An example of a suitable test method for determining the stripping performance of a ribbon is disclosed in the article by Mills, G., "Testing of 4- and 8-fiber ribbon strippability", 472 International Wire & Cable Symposium Proceedings (1992), the complete disclosure of which is incorporated herein by reference.
Many attempts have been made to understand the problems associated with ribbon stripping and to find a solution to increase ribbon stripping performance. The following publications attempt to explain and solve the problems associated with ribbon stripping: K. W. Jackson, et. al., "The Effect of Fiber Ribbon Component Materials on Mechanical and Environmental Performance", 28 International Wire & Symposium Proceedings (1993); H. C. Chandon, et. al., "Fiber Protective Design for Evolving Telecommunication Applications", International Wire & Symposium Proceedings (1992); J. R. Toler, et. al., "Factors Affecting Mechanical Stripping of Polymer Coatings From Optical Fibers", International Wire & Cable Symposium Proceedings (1989); and W. Griffioen, "Strippability of Optical Fibers", EFOC & N, Eleventh Annual Conference, Hague (1993).
The ability of a ribbon assembly to ribbon strip cleanly so as to provide bare optical glass fibers that are substantially free of residue is still unpredictable and the factors affecting ribbon stripping are not fully understood. There is still a need for an understanding of how the problems of ribbon stripping occur and a solution to these problems.